Gas Water Heating

CSI NUMBER

DEFINITION:

Gas water heating for domestic hot water can be engineered to provide space heating as well as water heating. Known as a combination( or combo) system, a single appliance can perform two functions.

CONSIDERATIONS:

There are several positive features to a combination water heating/space heating (combo) system. Several high efficiency models are available that allow venting of the water heaters through a wall instead of a chimney. Electric heating is not involved permitting a reduced electric panel size. Less space is needed compared to two separate systems. The air handler can be located to minimize ductwork.

The combo system circulates hot water from the water heater through a heat exchanger in the air handler. A blower will move the heated air through a standard duct system. In the summer, an air conditioner is connected to the exchanger and the system functions similarly, with cool air being pushed through the ductwork.

The use of gas water heaters is generally considered a positive (although not perfect) environmentally-based choice. The pollutant levels created by natural gas combustion are less than other conventional water heating options except solar.

Commercial
Status

Implementation
Issues

T
E
C
H
N
O
L
O
G
Y

S
U
P
P
L
I
E
R
S

C
O
S
T

F
I
N
A
N
C
I
N
G

A
C
C
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P
T
A
N
C
E

R
E
G
U
L
A
T
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R
Y

Gas Water Heating

Gas Space/Water Heating

Satisfactory

Satisfactory in most conditions

Satisfactory in Limited Conditions

Unsatisfactory or Difficult

COMMERCIAL STATUS

TECHNOLOGY:

Well-developed.

SUPPLIERS:

Plentiful.

COST:

Competitive with other conventional options such as heat pumps and electric water heaters. Rebates are also available from the City of Austin for the installation of these systems.

IMPLEMENTATION ISSUES

FINANCING:

None.

PUBLIC ACCEPTANCE:

Generally accepted; low operating costs desirable; not well-known.

REGULATORY:

Must meet applicable codes.

GUIDELINES

1.0 System Selection for Combination Systems

The selection of a combination system is based on sizing the combination system to the heat demand of the house, according to the following method:

A Manual J (ACCA) calculation, or comparable analysis, is conducted to determine the design heat loss of the building.

An air handler and its coil is selected to have a capacity that is equal to or greater than the design heat loss.

The water heater output must equal or exceed the rated output of the coil and air handler.

Water heaters are rated by their input (in BTUH). Thus, the water heater is selected by dividing the air handler/coil heating capacity by the recovery efficiency of the water heater. To qualify for a rebate from the City of Austin, the minimum recovery efficiency must be 76%. To qualify for a rating in the Green Builder Program, the minimum recovery efficiency is 80% and the Energy Factor must be 0.60 or greater (Recovery Efficiencies and Energy Factors are found in the GAMA directory listed in the Resources Section).

The minimum tank size for the system should be 40 gallons.

The design heat gain for the building is determined using Manual J.

The cooling coil capacity and air conditioner must meet or exceed the design heat gain.

Unless special hot water demand such as a Jacuzzi exist in a home, the water heater sized to meet the space heating demand will meet the domestic hot water needs of the household. No extra tank or heat capacity is needed.

Example: The design heat loss for a house is 26,000 BTUH and the design heat gain is 23,000 BTUH.

The design cooling load requires 2 tons of cooling or 24,000 BTUH.

An air handler is selected that has 30,000 BTUH heating capacity and 24,000 BTUH cooling capacity. (The air handler heating capacity appropriately exceeds the design heat loss.)